Light-based erase bars are used for discharging photoconductor belts in electrophotographic imaging systems. In multi-color electrophotographic imaging systems, latent images are formed in an imaging region of a moving photoconductor belt. Each of the latent images is representative of one of a plurality of different color separation images. The color separation images together define an overall multi-color image. The color separation images may define, for example, yellow, magenta, cyan, and black components that, upon subtractive combination on output media, produce a visible representation of the multi-color image. Prior to an imaging cycle, any charge remaining on the surface or in the photoconductor from a previous imaging operation must be uniformly discharged, typically by using a light-based erase bar. A uniform charge is next applied to the surface of the photoconductor.
Each of the latent images is formed by scanning a modulated laser beam across the moving photoconductor to selectively discharge the photoconductor in an image-wise pattern. Appropriately colored developers are applied to the photoconductor after each latent image is formed to develop the latent images. The resulting color separation images ultimately are transferred to the output media to form the multi-color image.
In some electrophotographic imaging systems, the latent images are formed and developed on top of one another in a common imaging region of the photoconductor. The latent images can be formed and developed in multiple passes of the photoconductor around a continuous transport path (i.e., a multi-pass system). Alternatively, the latent images can be formed and developed in a single pass of the photoconductor around the continuous transport path. A single-pass system enables multi-color images to be assembled at extremely high speeds relative to the multi-pass system. An example of an electrophotographic imaging system configured to assemble a multi-color image in a single pass of a photoconductor is disclosed in co-pending U.S. patent application Ser. No. 08/537,296 to Kellie et al., filed Sep. 29, 1995, and entitled "METHOD AND APPARATUS FOR PRODUCING A MULTI-COLORED IMAGE IN AN ELECTROPHOTOGRAPHIC SYSTEM".
In an electrophotographic imaging system as described above, the electrophotographic process requires several steps to be performed in a cyclic fashion to reproduce the images. Performance of these process steps can leave the photoconductor in a state of electric charge distribution that affects subsequent process cycles. This may result in previously processed images showing up as part of subsequent images, typically referred to as ghost images. In addition, accumulated charge at localized sites on the photoconductor can generate extremely high local electric fields that create regions of non-uniform conductivity and even dielectric breakdown, thus shortening the life time of the photoconductor.
By incorporating an erase step into the process, any charge remaining on the surface or trapped within the photoconductor from the previous imaging operation is uniformly discharged. The result is that the electric charge distribution induced from the previous imaging and developing cycle is uniformly reduced, reducing ghosting and increasing the life of the photoconductor belt.
The erase step is typically accomplished using any of a variety of "erase bars". In one approach, a linear array of light emitting diodes is used as a light source for removing charge from the photoconductor. The diodes are selected so that their emission wavelength matches the spectral response of the photoconductor (i.e., photoreceptor). The diodes typically are required to be spaced every inch or closer, depending on their operating characteristics. Because of diode-to-diode differences in operating characteristics, a "balancing" circuit is often times necessary to tune the current to each diode to achieve a more uniform electrophotographic discharge or erase. In addition, other circuitry is often required to detect single diode failures. Undetected failures of one or more light emitting diodes often leads to linear defects in the images that appear with continued cycling.
In another approach, a broad band linear incandescent source is used as an erase bar. However, this approach often introduces unwanted heat into the electrophotographic imaging system. In another approach, a fluorescent light source may be used. Both incandescent and fluorescent light sources may introduce unwanted wavelengths which cause photochemical reactions in the photoreceptors. While these photochemical reactions can be filtered against, the use of filtering devices results in additional costs to the electrophotographic imaging system.
In the above electrophotographic imaging system, the latent images must be formed in precise registration with one another to produce a high quality color image. In systems incorporating a photoconductor belt, precise registration can be difficult due to deviation of the belt from a transport path in a direction perpendicular to the transport path. Specifically, the photoconductor belt can undergo side-to-side movement (i.e., belt walking) during travel. The imaging region in which the latent images are formed is commonly fixed relative to the edge of the photoconductor belt. However, the scanning beam used to form each latent image on the imaging region is fixed relative to a start-of-scan coordinate. The side-to-side movement or belt walking of the photoconductor belt can cause movement of the imaging region relative to the start-of-scan coordinate. As a result, misregistration can occur between different scan lines and between different latent images. This misregistration can significantly degrade image quality. In particular, the misregistration can produce visible artifacts in the final multi-color image upon transfer of the misregistered color separation images to the output media.
Solutions to eliminate misregistration (i.e., belt misregistration and/or image misregistration) which are incorporated into electrophotographic imaging systems often require an additional optical source and power supply to detect the side-to-side movement of the photoconductor belt. The use of additional components however may not be desirable because they add cost and complexity to the electrophotographic imaging system. The use of additional components may also reduce overall system reliability.